Dry solids of anionically modified cellulose nanofibers and processes for preparing them

ABSTRACT

Dry solids of anionically modified cellulose nanofibers with good redispersion are provided by incorporating 5 to 300% by mass of a water-soluble polymer relative to the anionically modified cellulose nanofibers during the preparation of the dry solids of anionically modified cellulose nanofibers.

This application is the U.S. national phase of International ApplicationNo. PCT/JP2015/050512 filed 9 Jan. 2015 which designated the U.S. andclaims priority to JP Patent No. 2014-006445 filed 17 Jan. 2014, theentire contents of each of which are hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to dry solid of anionically modifiedcellulose nanofibers.

BACKGROUND ART

Anionically modified cellulose nanofibers (CNFs) are fine fibers havinga fiber diameter in the order of about 4 to several hundred nanometerswith high dispersion in water systems so that they are expected to beapplied for maintaining the viscosity of foods, cosmetics, medicalproducts or coatings or the like; for strengthening dough and allowingit to retain water; for improving food stability; or for serving aslow-calorie additives or emulsion stabilizing aids. Once the anionicallymodified CNFs dispersed in water (i.e., in the wet state) are dried intodry solids, they would not recover various properties such assolubility, dispersity, the degree of precipitation, and viscosity tolevels comparable to those they exhibited before drying (i.e., in thewet state) even if one tried to redisperse the dry solids by addingwater because hydrogen bonds are formed between the fine cellulosefibers. For this reason, the anionically modified CNFs have beenprepared in the state where they are dispersed in water (in the wetstate) and normally used in the wet state without being dried forvarious applications.

To stably maintain the anionically modified CNFs in the wet state,however, a several times to several hundred times greater mass of wateris required relative to the anionically modified CNFs, which causesvarious problems such as a need for a large storage space, an increasein storage and shipping costs and the like. Some means for solving theseproblems have been proposed, including freeze-drying, critical pointdrying, drying after displacement with an organic solvent (patentdocument 1) and the like.

CITATION LIST Patent Documents

Patent document 1: JPA No. 1994-233691.

SUMMARY OF INVENTION Technical Problem

However, freeze-drying anionically modified CNFs requires enormousamounts of energy, and may cause problems such as growth of ice crystalslarger than gaps between fine fibers of the anionically modified CNFs toresult in mutual association of the fine fibers of the anionicallymodified CNFs when water freezes between the fine fibers under someconditions.

On the other hand, drying by solvent displacement requires a greatamount of solvent and time because gaps between fine fibers ofanionically modified CNFs are very small and a lot of water occurs onthe surfaces of the fine cellulose fibers. In addition, the surfaces ofthe fine cellulose fibers of the anionically modified CNFs firmly adhereto each other via hydrogen bonds during the drying process of thesolvent because water that cannot be displaced by the solvent remains.This makes it difficult for the anionically modified CNFs to restore theoriginal state when they are redispersed.

Thus, the present invention aims to provide dry solids of anionicallymodified CNFs with good redispersion. The expression “good redispersion”means that the dry solids of anionically modified CNFs show littlechanges in their properties such as solubility and dispersity as well asthe degree of precipitation, viscosity and transparency when they aredispersed, as compared with the anionically modified CNFs before drying.

Solution to Problem

The problems described above are solved by means including [1] to [6]below:

-   [1] A dry solid of an anionically modified cellulose nanofiber,    comprising 5 to 300% by mass of a water-soluble polymer relative to    the anionically modified cellulose nanofiber.-   [2] The dry solid of an anionically modified cellulose nanofiber as    defined in [1], wherein the anionically modified cellulose nanofiber    is a carboxylated cellulose nanofiber having a carboxyl group    content of 0.6 mmol/g to 2.0 mmol/g based on the bone dry mass of    the anionically modified cellulose nanofiber.-   [3] The dry solid of an anionically modified cellulose nanofiber as    defined in [1], wherein the anionically modified cellulose nanofiber    is a carboxymethylated cellulose nanofiber having a degree of    carboxymethyl substitution of 0.01 to 0.50 per glucose unit of the    anionically modified cellulose nanofiber.-   [4] The dry solid of an anionically modified cellulose nanofiber as    defined in any one of [1] to [3], wherein the water-soluble polymer    is a carboxymethyl cellulose and a salt thereof.-   [5] A process for preparing the dry solid of an anionically modified    cellulose nanofiber as defined in any one of [1] to [4], comprising    forming a thin film of an aqueous suspension containing the    anionically modified cellulose nanofiber and a water-soluble    polymer, and drying the thin film.-   [6] A process for preparing the dry solid of an anionically modified    cellulose nanofiber as defined in any one of [1] to [4], comprising    adjusting an aqueous suspension containing the anionically modified    cellulose nanofiber and a water-soluble polymer to pH 9 to 11, and    then drying it.

Advantageous Effects of Invention

The present invention makes it possible to provide dry solids ofanionically modified CNFs with good redispersion. The expression “goodredispersion” means that the dry solids of anionically modified CNFsshow little changes in their properties such as solubility anddispersity as well as the degree of precipitation, viscosity andtransparency when they are dispersed, as compared with the anionicallymodified CNFs before drying.

DESCRIPTION OF EMBODIMENTS

The dry solids of anionically modified cellulose nanofibers of thepresent invention obtained by incorporating 5 to 300% by mass of awater-soluble polymer relative to the anionically modified CNFs arecharacterized in that they show little changes in their properties suchas solubility, dispersity, the degree of precipitation, and viscositywhen the solids are redispersed in water back into dispersions of theanionically modified CNFs in the wet state, as compared with thedispersions of the anionically modified CNFs in the wet state beforedrying (good redispersion).

The reason why the dry solids of anionically modified CNFs of thepresent invention exhibit excellent redispersion is not exactly known,but may be explained as follows: Anionically modified CNFs have electriccharges on their surfaces so that they undergo electrostatic repulsionand stably exist in water while they are also redispersive to someextent when they are dried. However, the electric charges on thesurfaces of the anionically modified CNFs are not homogeneous, andhydrogen bonds are formed to cause aggregation in regions with lowcharge density when drying, whereby sufficient redispersion cannot beachieved. If the anionically modified CNFs are combined with awater-soluble polymer, however, the water-soluble polymer covers theregions with low charge density on the surfaces of the anionicallymodified CNFs to prevent the formation of hydrogen bonds, wherebysufficient redispersion can be achieved.

As used herein, the term “dry solids of anionically modified cellulosenanofibers” refers to the anionically modified cellulose nanofibersdried to a moisture content of 12% by mass or less.

(Anionically Modified Cellulose Nanofibers)

As used herein, the term “anionically modified cellulose nanofibers(anionically modified CNFs)” refers to fine fibers having a fiber widthof about 4 to 500 nm and an aspect ratio of 100 or more that call beobtained by defibrating an anionically modified cellulose such as acarboxylated cellulose (also referred to as oxidized cellulose),carboxymethylated cellulose, phosphate ester group-containing celluloseor the like.

(Cellulose Base Materials)

Cellulose base materials for preparing the anionically modifiedcellulose include, for example, those derived from plant materials(e.g., wood, bamboo, hemp, jute, kenaf, farm wastes, cloth, pulp(softwood unbleached kraft pulp (NUKP), softwood bleached kraft pulp(NBKP), hardwood unbleached kraft pulp (LUKP), hardwood bleached kraftpulp (LBKP), softwood unbleached sulfite pulp (NUSP), softwood bleachedsulfite pulp (NBSP), thermomechanical pulp (TMP), recycled pulp, wastepaper, etc.), animal materials (e.g., Ascidiacea), algae, microorganisms(e.g., acetic acid bacteria (Acetobacter)), microorganism-producingproducts and the like, and any of them can be used. Preferably used arecellulose fibers derived from plants or microorganisms, more preferablycellulose fibers derived from plants.

(Carboxymethylation)

When a carboxymethylated cellulose is used as an anionically modifiedcellulose in the present invention, the carboxymethylated cellulose maybe obtained by carboxymethylating any one of the cellulose basematerials listed above by a known method, or may be commerciallyavailable. In either case, it preferably has a degree of carboxymethylsubstitution of 0.01 to 0.50 per anhydrous glucose unit of thecellulose. An example of a process for preparing such acarboxymethylated cellulose is as follows. A cellulose is used as a basematerial in a solvent consisting of water and/or a lower alcohol in anamount of 3 to 20 times the mass of the base material, specificallyincluding water, methanol, ethanol, N-propyl alcohol, isopropyl alcohol,N-butanol, isobutanol, tert-butanol or the like alone or as a solventmixture of two or more of them. When a mixture containing a loweralcohol is used, the proportion of the lower alcohol is 60 to 95% bymass. A mercerizing agent consisting of an alkali metal hydroxide isused in an amount of 0.5 to 20 molar equivalents per anhydrous glucoseresidue of the base material, specifically including sodium hydroxide orpotassium hydroxide. The base material, solvent, and mercerizing agentare mixed to perform a mercerization process at a reaction temperature 0to 70° C., preferably 10 to 60° C. for a reaction period of 15 minutesto 8 hours, preferably 30 minutes to 7 hours. Then, a carboxymethylatingagent is added in an amount of 0.05 to 10.0 molar equivalents perglucose residue to perform an etherification reaction at a reactiontemperature of 30 to 90° C., preferably 40 to 80° C. for a reactionperiod of 30 minutes to 10 hours, preferably 1 hour to 4 hours.

As used herein, the term “carboxymethylated cellulose” as a kind ofanionically modified celluloses used for the preparation of anionicallymodified CNFs refers to the one that retains at least partially itsfibrous morphology even when it is dispersed in water. Therefore, it isdistinguished from carboxymethyl cellulose that is a kind ofwater-soluble polymers described herein later. When an aqueousdispersion of a “carboxymethylated cellulose” is analyzed by electronmicroscopy, fibrous substances can be observed. However, any fibroussubstances are not observed in an aqueous dispersion of a carboxymethylcellulose, which is a kind of water-soluble polymers. Moreover, peaks oftype I cellulose crystals can be observed in a “carboxymethylatedcellulose” by X-ray diffraction, but type I cellulose crystals cannot befound in a carboxymethyl cellulose used as a water-soluble polymer.

(Carboxylation)

When a carboxylated (oxidized) cellulose is used as an anionicallymodified cellulose in the present invention, the carboxylated cellulose(also referred to as oxidized cellulose) can be obtained bycarboxylating (oxidizing) any one of the cellulose base materials by aknown method. During the carboxylation, the carboxyl group content ispreferably adjusted to, but not specifically limited to, 0.6 to 2.0mmol/g, more preferably 1.0 mmol/g to 2.0 mmol/g based on the bone drymass of the anionically modified cellulose nanofiber.

An example of a carboxylation (oxidation) method comprises oxidizing acellulose base material using an oxidizing agent in water in thepresence of a compound selected from the group consisting of an N oxylcompound, a bromide, an iodide or a mixture thereof. This oxidationreaction allows the primary hydroxyl group at the C6 position of theglucopyranose ring on the surface of the cellulose to be selectivelyoxidized to give a cellulose fiber having an aldehyde group and acarboxyl group (—COOH) or a carboxylate group (—COO⁻) on its surface.During the reaction, the concentration of the cellulose is notspecifically limited, but preferably 5% by mass or less.

The term “N-oxyl compound” refers to a compound capable of generatingnitroxyl radicals. Any N-oxyl compounds can be used so far as theypromote an intended oxidation reaction. For example, they include2,2,6,6-tetramethylpiperidin-1-oxy radical (TEMPO) and derivativesthereof (e.g., 4-hydroxy-TEMPO).

The amount of the N-oxyl compound used is not specifically limited sofar as it is a catalytic amount enough to oxidize the cellulose used asa base material. For example, it is preferably 0.01 to 10 mmol, morepreferably 0.01 to 1 mmol, still more preferably 0.05 to 0.5 mmol pergram of bone dry cellulose. It is also preferably about 0.1 to 4 mmol/Lrelative to the reaction system.

The term “bromide” refers to a compound containing bromine, examples ofwhich include alkali metal bromides that can be ionized by dissociationin water. Similarly, the term “iodide” refers to a compound containingiodine, examples of which include alkali metal iodides. The amount ofthe bromide or iodide used can be selected in a range that can promotethe oxidation reaction. The total amount of the bromide and iodide ispreferably, for example, 0.1 to 100 mmol, more preferably 0.1 to 10mmol, still more preferably 0.5 to 5 mmol per gram of bone drycellulose.

Any known oxidizing agents can be used, including, for example,halogens, hypohalous acids, halous acids, perhalic acids or saltsthereof, halogen oxides, peroxides and the like. Among others, sodiumhypochlorite is preferred because it is inexpensive and has a lowenvironmental impact. The suitable amount of the oxidizing agent used ispreferably 0.5 to 500 mmol, more preferably 0.5 to 50 mmol, still morepreferably 1 to 25 mmol, most preferably 3 to 10 mmol per gram of bonedry cellulose, for example. It is also preferably 1 to 40 mol per moleof the N-oxyl compound, for example.

During the oxidation process of the cellulose, the reaction efficientlyproceeds even under relatively mild conditions. Thus, the reactiontemperature is preferably 4 to 40° C., or may be room temperature around15 to 30° C. As the reaction proceeds, the pH of the reaction solutionis found to decrease because carboxyl groups are generated in thecellulose. To ensure that the oxidation reaction efficiently proceeds,an alkaline solution such as an aqueous sodium hydroxide solution ispreferably added to maintain the pH of the reaction solution in theorder of 8 to 12, preferably 10 to 11. The reaction medium is preferablywater because of easy handling, low likelihood of side reactions and thelike.

The reaction period in the oxidation reaction can be appropriatelyselected depending on the extent to which oxidation proceeds, typicallyin the order of 0.5 to 6 hours, for example, 0.5 to 4 hours.

In addition, the oxidation reaction may be performed in two stages. Forexample, the oxidized cellulose obtained by filtration after thetermination of a first stage reaction can be oxidized again under thesame or different reaction conditions so that efficient oxidation can beachieved while preventing the reaction from being inhibited by sodiumchloride produced as a by-product during the first stage reaction.

Another example of a carboxylation (oxidation) method may comprisecontacting a cellulose base material with an ozone-containing gas. Thisoxidation reaction allows the hydroxyl groups at least at the 2- and6-positions of the glucopyranose ring to be oxidized and the cellulosechain to be cleaved. The ozone concentration in the ozone-containing gasis preferably 50 to 250 g/m³, more preferably 50 to 220 g/m³. The amountof ozone to be added to the cellulose base material is preferably 0.1 to30 parts by mass, more preferably 5 to 30 parts by mass per 100 parts bymass of the solids content of the cellulose base material. The ozonationtemperature is preferably 0 to 50° C., more preferably 20 to 50° C. Theozonation period is not specifically limited, but in the order of 1 to360 minutes, preferably 30 to 360 minutes. If the ozonation conditionsare within these ranges, the cellulose can be prevented from excessivelyoxidized and cleaved and an oxidized cellulose can be obtained with goodyield. The ozonation may be followed by a post-oxidation using anoxidizing agent. The oxidizing agent used for the post-oxidation is notspecifically limited, but may include chlorine compounds such aschlorine dioxide and sodium chlorite; and oxygen, hydrogen peroxide,persulfuric acid, peracetic acid and the like. For example, apost-oxidation can be performed by dissolving one of these oxidizingagents in water or a polar organic solvent such as an alcohol to preparea solution of the oxidizing agent and immersing the cellulose basematerial in the solution.

The carboxyl group content of the oxidized cellulose can be adjusted bycontrolling the reaction conditions such as the amount of the oxidizingagent added, the reaction period and the like described above.

(Defibration)

Equipment that can be used for defibrating the anionically modifiedcellulose may include, but not specifically limited to, high speedrotary machines, colloid mill machines, high pressure machines, rollmill machines, ultrasonic machines and the like. During the defibration,a high shear force should preferably be applied to an aqueous dispersionof the anionically modified cellulose. Especially for efficientdefibration, a wet type high pressure or ultra-high pressure homogenizercapable of applying a pressure of 50 MPa or more and a high shear forceto the aqueous dispersion should preferably be used. The pressure ismore preferably 100 MPa or more, still more preferably 140 MPa or more.If desired, the aqueous dispersion may be pretreated by using knownmixing, stirring, emulsifying or dispersing equipment such as a highspeed shear mixer prior to the defibration and dispersion process in thehigh pressure homogenizer.

(Water-Soluble Polymers)

Water-soluble polymers that can be used in the present inventioninclude, for example, cellulose derivatives (carboxymethyl cellulose,methyl cellulose, hydroxypropyl cellulose, ethyl cellulose), xanthangum, xyloglucan, dextrin, dextran, carrageenan, locust bean gum, alginicacid, alginates, pullulan, starches, potato starch, arrowroot starch,processed starches (cationized starch, monostarch phosphate, distarchphosphate, phosphated distarch phosphate, hythoxypropyl starch,hydroxypropyl distarch phosphate, acetylated distarch adipate,acetylated distarch phosphate, acetylated oxidized starch, starch sodiumoctenylsuccinate, starch acetate, oxidized starch), corn starch, gumarabic, locust bean gum, gellan gum, polydextrose, pectin, chitin,water-soluble chitin, chitosan, casein, albumin, soy proteinhydrolysate, peptone, polyvinyl alcohol, polyacrylamide, sodiumpolyacrylate, polyvinylpyrrolidone, polyvinyl acetate, polyamino acids,polylactic acid, polymalic acid, polyglycerin, latex, rosin sizingagents, petroleum resin sizing agents, urea resins, melamine resins,epoxy resins, polyamide resins, polyamide-polyamine resins,polyethyleneimines, polyamines, plant gums, polyethylene oxide,hydrophilic crosslinked polymers, polyacrylates, starch-polyacrylatecopolymers, tamarind gum, guar gum and colloidal silica and mixtures ofone or more of them. Among them, cellulose derivatives are preferredbecause of their compatibility for anionically modified cellulosenanofibers, among which especially preferred are carboxymethylcelluloses and salts thereof. Water-soluble polymers such ascarboxymethyl celluloses and salts thereof are thought to improveredispersion by entering between anionically modified cellulosenanofibers to expand the distance between the CNFs.

When a carboxymethyl cellulose or a salt thereof is used as awater-soluble polymer, the carboxymethyl cellulose preferably has adegree of carboxymethyl substitution of 0.55 to 1.6, more preferably0.55 to 1.1, still more preferably 0.65 to 1.1 per anhydrous glucoseunit. In addition, it preferably has a longer molecular length (a higherviscosity) to more effectively expand the distance between CNFs, andtherefore, it preferably has a Brookfield viscosity of 3 to 14000 mPa·s,more preferably 7 to 14000 mPa·s, still more preferably 1000 to 8000mPa·s in a 1% by mass aqueous solution of the carboxymethyl cellulose at25° C., 600 rpm.

The amount of the water-soluble polymer added is 5 to 300% by mass,preferably 20 to 300% by mass relative to the anionically modified CNF(bone dry solids). If it is less than 5% by mass, sufficientredispersion cannot be achieved, but if it exceeds 300% by mass,characteristic features of anionically modified CNFs including viscositycharacteristics such as thixotropy and dispersion stability areimpaired.

The amount of the water-soluble polymer added is preferably 25% by massor more because especially excellent redispersion can be achieved. Interms of thixotropy, it is preferably 200% by mass or less, especiallypreferably 60% by mass or less.

(Processes for Preparing the Dry Solids)

The dry solids of anionically modified cellulose nanofibers with goodredispersion can be obtained by drying an aqueous suspension containingan anionically modified cellulose nanofiber and a water-soluble polymer.Here, drying preferably takes place after the pH is adjusted 9 to 11,because dry solid of anionically modified cellulose nanofibers with moreimproved redispersion can be obtained.

(Drying Methods)

Known drying methods can be used, including, for example, spray drying,squeezing, air drying, hot air drying, and vacuum drying. Dryers thatcan be used include, but not specifically limited to, continuous tunneldryers, band dryers, vertical dryers, vertical turbo dryers, multistagedisc dryers, ventilation dryers, rotary dryers, pneumatic conveyingdryers, spray dryers, atomizing dryers, cylinder dryers, drum dryers,belt dryers, screw conveyor dryers, rotary dryers with heating tubes,oscillating conveyor dryers, batch tray dryers, ventilation dryers,vacuum tray dryers, and agitated dryers and the like, and they can beused alone or as a combination of two or more of them.

Among them, dryers by which a thin film is formed and dried arepreferably used in terms of energy efficiency because heat energy can bedirectly and uniformly supplied to an object to be dried so that dryingcan take place more efficiently and rapidly. Such dryers by which a thinfilm is formed and dried are also preferred because the dried object canbe immediately collected by simple means such as scraping off the thinfilm. Moreover, it was found that redispersion is further improved whena thin film is formed once and dried. Examples of dryers by which a thinfilm is formed and dried include, for example, drum dryers and beltdryers by which a thin film is formed on a drum or a belt by a blade ora die or the like and then dried.

The thin film to be dried preferably has a thickness of 50 to 1000 μm,more preferably 100 to 300 μm. Thin films having a thickness of 50 μm ormore are easily scraped off after drying, and those having a thicknessof 1000 μm or less exhibit further improved redispersion.

(Dry Solids of Anionically Modified CNFs)

The dry solids of anionically modified CNFs of the present invention canbe used as additives in various fields generally using additives such asfoods, beverages, personal care products, cosmetics, medicaments,various chemical products, papermaking, civil construction, coatings,inks, coating compositions, agrochemicals, architecture, automobiles,chemicals for epidemic control, electronic materials, batteries, flameretardants, heat insulating materials, household goods, cleaning agents,water treatment, drilling fluids, neutral functional materials, spillcontrol and/or recovery of shale gas and oil. Specifically, they can beused as thickeners, gelling agents, glues, food additives, excipients,reinforcing materials for rubbers and plastics, additives for coatings,additives for adhesives, additives for papermaking, polishing agents,water absorbing materials, deodorants, rust inhibitors, water retentionagents, moisturizing agents, refrigerating agents, structuring agents,mud rheology modifier, filter aids and mud overflow inhibitors and thelike, and further can be applied in rubber/plastic materials, coatings,adhesives, coatings for coated papers, coated papers, binders,cosmetics, lubricating compositions, polishing compositions, wrinklereducers for clothing, gliding agents for ironing and the likecontaining these agents as components.

The dry solids of anionically modified CNFs of the present inventionhave desirable rheological properties, specifically thixotropy,desirable yield stress, reversible gelation, i.e., the property ofreturning to gels by still standing even if a shear force is applied,and temperature-insensitive elasticity so that they can preferably beused for applications taking advantages of these characteristics.

In food applications, the dry solids of anionically modified CNFs of thepresent invention can be used as, for example, rheology modifiers. Forexample, they can be used as stabilizers against creaming orprecipitation in suspensions, or as indigestible dietary fibers.

In personal care products and household products, the dry solids ofanionically modified CNFs can be used, for example, for stabilizingemulsions, suspensions, dispersions and foams. For example, they can beused in creams, lotions and gels for application to the skin;sunscreens; cleaning soaps or gels; antiperspirants and deodorants inthe form of sticks, pump sprays, aerosols or roll-ons; fragrance releasegels; lip colors, lip glosses, liquid cosmetic products; toothpastes,tooth cleaning agents; oral care products such as denture care productsincluding whitening agents, cleansers and adhesives; controlled-release,sustained-release or delayed-release products; ointments, anesthetics,preservatives; wound care products such as dressings; absorbent productssuch as diapers; bleaching agents; shampoos; air cleaning agents;cleaning agents in the form of liquids, gels, pastes and foams(detergents, spot cleaners, softeners, etc.); aqueous or gel-like toiletbowl cleaners and the like.

In medical applications, the dry solids of anionically modified CNFs canbe used as, for example, agents for controlling, sustaining or delayingthe release of drugs. They can also be used as disintegrants fortablets, dietary fibers, liquid retention agents in wound care productsand the like, as well as rheology modifiers.

In papermaking and film coating processes, the dry solids of anionicallymodified CNFs of the present invention can be used as emulsionmodifiers, stabilizers, sizing agents, component separation inhibitors,component precipitation inhibitors, clarifying agents, absorbents,drainage aids, molding aids, flocculation aids, deposit or scaleinhibitors, water treatment agents, dewatering agents, film- andskin-forming aids, crosslinking agents for polymer electrolytes,additives for eliminating hazardous substances, paper coating agents,hardness improvers, wet strength improvers, odor absorption improvers,flexibility improvers, strength improvers, tear resistance improvers,gas barrier improvers, and bending resistance improvers and the like.

In papermaking, scale control refers to the inhibition of deposition ofcalcium carbonate and calcium oxalate formed during pulping. Scalecontrol can be achieved by dispersing salt crystals in media, or byinhibiting nucleation, or by inhibiting the formation of crystals thatwill be deposited. The anionically modified cellulose nanofibers havinga very small size and stabilized by introducing functional groups seemto help inhibit the growth of crystals that will be deposited andthereby inhibit scale deposition.

The dry solids of anionically modified CNFs can also be used inpapermaking machines to increase the drainage and/or dewatering speedduring papermaking; to retain dispersed organic and/inorganic particles(such as pulp powder, fillers, sizing agents, dyes, and/or clay); toretain hazardous organic and inorganic particulate matters; to improvethe uniformity of paper sheets formed; and to improve the strength ofpaper sheets. Drainage aids are additives for increasing the speed atwhich water is drained from paper slurry on papermaking machines. Suchadditives allow rapider paper formation and therefore increaseprofitability. The anionically modified cellulose nanofibers have thefunction of increasing the drainage speed.

The dry solids of anionically modified CNFs can also be used to controlthe flow of color coatings, and to provide water retention properties sothat they can be used in the field of coated paper. This allows theamount of water penetration into base paper to be controlled.

In coating compositions such as coatings and inks, the dry solids ofanionically modified CNFs can be used as rheology modifiers, anti-splashagents, leveling agents, anti-drip agents, water penetration resistanceimprovers, dispersants and the like. Especially, they are useful for gelcoatings. In inks such as inkjet printing inks, they can be used as dyedispersants and/or stabilizers, charge control agents, or flow controlagents.

In the field of water treatment, the dry solids of anionically modifiedCNFs can be used as scale control agents, i.e., inhibitors againstformation and/or increase of inorganic deposits in aqueous systems,clarifying agents, flocculation aids, precipitants, flocculants, chargetransfer agents, and softeners.

In drilling fluids, the dry solids of anionically modified CNFs can beused as rheology modifiers to confer thixotropy on drilling fluids sothat they have low viscosity during cutting but otherwise highviscosity. This reduces loss of drilling fluids and facilitates recoveryof used drilling fluids.

In agricultural applications, the dry solids of anionically modifiedCNFs of the present invention can be used for soil treatment so thatthey can confer water retention, corrosion resistance, and freezeresistance on the soil. They can also be used as agents for controlledrelease, sustained release or delayed release of agrochemicals such asfertilizers, insecticides, fungicides and herbicides. They can also beused as crop protection agents for minimizing or preventing frostdamage.

In the field of architecture, the dry solids of anionically modifiedCNFs can be used in concrete dispersants, drywall mud, reinforcingmaterials in construction caulks, water-soluble adhesives, and boardmaking.

In the field of reinforcing materials, the dry solids of anionicallymodified CNFs can be used in rubbers (NR, SBR, EPDM, NBR, etc.) andplastics (polyolefin resins, acrylic resins, urethane resins, PVCresins, polyamide resins and PC resins, etc.).

EXAMPLES

The following examples further illustrate the present invention without,however, limiting the present invention thereto.

<Preparation of a Carboxylated (TEMPO-Oxidized) CNF>

To 500 ml of an aqueous solution of 780 mg of TEMPO (from Sigma Aldrich)and 75.5 g of sodium bromide dissolved therein was added 500 g (bonedry) of an unbeaten softwood bleached kraft pulp (brightness 85%), andthe mixture was stirred until the pulp was homogeneously dispersed. Anaqueous sodium hypochlorite solution was added to the reaction system inan amount of 6.0 mmol/g to start an oxidation reaction. During thereaction, the pH in the system decreased, and a 3M aqueous sodiumhydroxide solution was sequentially added to adjust the reaction systemto pH 10. The reaction was terminated when sodium hypochlorite has beenconsumed and the pH in the system showed no change. After the reaction,the mixture was filtered through a glass filter to separate the pulp,and the pulp was thoroughly washed with water to give an oxidized pulp(carboxylated cellulose). The pulp yield was 90%, the time required forthe oxidation reaction was 90 minutes, and the carboxyl group contentwas 1.6 mmol/g.

The oxidized pulp obtained in the process described above was adjustedto 1.0% (w/v) with water, and treated in a ultra-high pressurehomogenizer (20° C., 150 Mpa) for three cycles to give an anionicallymodified cellulose nanofiber dispersion. The resulting fiber had anaverage fiber diameter of 40 nm and an aspect ratio of 150.

<Determination Method of the Carboxyl Group Content>

Sixty ml of a 0.5% by mass slurry (aqueous dispersion) of thecarboxylated cellulose was prepared and adjusted to pH 2.5 by adding a0.1 M aqueous hydrochloric acid solution, and then a 0.05 N aqueoussodium hydroxide solution was added dropwise while the electricconductivity was measured until the pH reached 11, and the carboxylgroup content was calculated from the amount of sodium hydroxide (a)consumed during the neutralization stage of the weak acid characterizedby a slow change in electric conductivity by the equation below:Carboxyl group content[mmol/g carboxylated cellulose]=a[ml]×0.05/mass[g]of carboxylated cellulose.

<Preparation of a Carboxymethylated (CM) CNF>

In a stirrer capable of mixing pulp, 200 g of a pulp (NBKP (softwoodbleached kraft pulp), from Nippon Paper Industries Co., Ltd.) on a drymass basis and 111 g of sodium hydroxide on a dry mass basis were addedand water was added in such an amount that the pulp had a solids contentof 20% (w/v). Then, the mixture was stirred at 30° C. for 30 minutes,and then 216 g (on an active matter basis) of sodium monochloroacetatewas added. The mixture was stirred for 30 minutes, and then warmed to70° C. and stirred for 1 hour. Then, the reaction mixture was removedand neutralized and washed to give a carboxymethylated pulp having adegree of carboxymethyl substitution of 0.25 per glucose unit. Then, thecarboxymethylated pulp was adjusted to a solids content of 1% withwater, and defibrated by 5 treatment cycles in a high pressurehomogenizer at 20° C., 150 MPa to give a carboxymethylated cellulosefiber. The resulting fiber had an average fiber diameter of 50 nm and anaspect ratio of 120.

<Determination Method of the Degree of Carboxymethyl Substitution PerGlucose Unit>

Precisely weighed about 2.0 g of the carboxymethylated cellulose fiber(bone dry) was added into a 300-mL Erlenmeyer flask with a ground glassstopper. To this was added 100 mL of a mixture of 100 mL of specialgrade concentrated nitric acid in 1000 mL of nitric acid-methanol, andthe mixture was agitated for 3 hours to convert the carboxymethylcellulose salt (CM cellulose.) into a hydrogen-substituted CM cellulose.Precisely weighed 1.5 to 2.0 g of the hydrogen-substituted CM cellulose(bone dry) was added into a 300-mL Erlenmeyer flask with a ground glassstopper. The hydrogen-substituted CM cellulose was wetted with 15 mL of80% methanol, and 100 mL of 0.1N NaOH was added, and the mixture wasagitated at room temperature for 3 hours. Using phenolphthalein as anindicator, the excess of NaOH was back-titrated with 0.1N H₂SO₄. Thedegree of carboxymethyl substitution (DS) was calculated by the equationbelow:A=[(100×F′−(0.1N H₂SO₄)(mL)×F)×0.1]/(bone dry mass(g) ofhydrogen-substituted CM cellulose)DS=0.162×A/(1−0.058×A)

-   A: the amount of 1N NaOH (mL) required for neutralizing 1 g of the    hydrogen-substituted CM cellulose-   F′: the factor of 0.1N H₂SO₄-   F: the factor of 0.1N NaOH.

<Determination Methods of the Average Fiber Diameter and Aspect Ratio>

Randomly chosen 200 fibers of the anionically modified CNFs wereanalyzed for their average fiber diameter and average fiber length usinga field emission scanning electron microscope (FE-SEM). It should benoted that the aspect ratio was calculated by the equation below:Aspect ratio=average fiber length/average fiber diameter.

Example 1

<Preparation of a Dry Solid>

The carboxylated CNF described above (carboxyl group content: 1.6mmol/g; average fiber diameter: 40 nm; aspect ratio: 150) was used as ananionically modified CNF, To a 0.7% by mass aqueous auspcnsiun of theanionically modified CNF was added a carboxymethyl cellulose (availableunder the brand name F350HC-4 having a viscosity of about 3000 mPa·s(1%, 25° C.), and a degree of carboxymethyl substitution of about 0.9)in an amount of 40% by mass relative to the anionically modified CNF(i.e., in such an amount that the carboxymethyl cellulose had a solidscontent of 4U parts by mass per 100 parts by mass of the solids contentof the anionically modified CNF), and the mixture was stirred by usingT.K. HOMO MIXER (12,000 rpm) for 60 minutes to prepare an aqueousdispersion of the anionically modified CNF. This dispersion had a pH ofabout 7 to 8. This aqueous dispersion was adjusted to pH 9 by adding0.5% of an aqueous sodium hydroxide solution, and then applied on thedrum surface of the drum dryer D0405 (from KATSURAGI INDUSTRY CO. LTD.)to form a thin film having a thickness of about 100 to 200 μm, and driedat a vapor pressure of 0.5 MPa·G and a drum rotation speed of 2 rpm togive a dry solid of the anionically modified CNF having a moisturecontent of 5% by mass

<Redispersion of the Dry Solid>

Then, the dry solid obtained as described above was redispersed in waterfor the purpose of evaluating the redispersion of the dry solid. Theredispersion was evaluated under two conditions using a high powerdisperser (vigorous stirring) and a low power disperser (mild stirring)to assess the extent of redispersion.

Water was added to the dry solid obtained as described above to preparea 0.7% by mass aqueous suspension, which was then vigorously stirredusing T.K. HOMO MIXER (12,000 rpm) for 60 minutes or mildly stirredusing a propeller agitator (600 rpm) for 3 hours to give an aqueousdispersion/suspension of the anionically modified CNF redispersedtherein.

<Evaluation of Dispersity>

The vigorously or mildly stirred aqueous dispersion/suspension of theanionically modified CNF was diluted to 0.1% and placed on a microscopeslide and evaluated for whether undispersed gel particles were foundwhen the microscope slide was inclined. Evaluation was made according tothe following scoring criteria: 3: no visible particles; 2: visibleparticles; 1: most of particles were undispersed and separated fromwater.

<Determination of Viscosity>

The Brookfield viscosity of the aqueous dispersion/suspension of theanionically modified CNF (solids content 0.7%, 25° C.) was measuredunder conditions of a rotation speed of 30 rpm for 3 minutes.

<Determination of Transparency>

The transparency (light transmittance at 660 nm) of the aqueousdispersion/suspension of the anionically modified CNF (solids content0.1%) was measured by using the UV spectrophotometer U-3000 (fromHitachi High-Technologies Corporation).

<Evaluation of Recovery Rates>

Changes in the viscosity and transparency of the dispersion/suspensionbefore and after drying and redispersion were evaluated as the recoveryrate of viscosity or transparency. The recovery rate was calculated bythe equation below:Recovery rate (%)=(viscosity or transparency of dispersion beforedrying)/(viscosity or transparency of dispersion/suspension afterredispersion)×100.

<Determination of the Thixotropy of the Dry Solid>

The Brookfield viscosity of the aqueous dispersion/suspension of theanionically modified CNF redispersed by vigorous stirring was measuredimmediately after stirring (rotation speed 30 rpm, 3 minutes) and afterstill standing for a day (rotation speed 30 rpm, 3 minutes), and theratio between both values were evaluated according to the followingscoring criteria: 4: the ratio was 2 or more; 3: the ratio was 1.5 ormore and less than 2; 2; the ratio was more than 1 and less than 1.5; 1:the ratio was 1.

Example 2

The same procedure as described in Example 1 was performed except thatthe pH was adjusted to 8 in the preparation of the dry solid.

Example 3

The same procedure as described in Example 1 was performed except thatthe pH was adjusted to 11 in the preparation of the dry solid.

Example 4

The same procedure as described in Example 1 was performed except thatthe carboxymethyl cellulose was added in an amount of 100% by massrelative to the anionically modified CNF in the preparation of the drysolid.

Example 5

The same procedure as described in Example 1 was performed except thatthe CM CNF obtained as described above (degree of carboxymethylsubstitution: 0.25; average fiber diameter: 50 nm; aspect ratio: 120)was used as an anionically modified CNF.

Example 6

The same procedure as described in Example 5 was performed except thatthe pH was adjusted to 8 in the preparation of the dry solid.

Example 7

The same procedure as described in Example 5 was performed except thatthe pH was adjusted to 11 in the preparation of the dry solid.

Examples 8 to 15

The same procedure as described in Example 5 was performed except thatthe carboxymethyl cellulose was added in an amount of 5, 20, 25, 65,100, 150, 200, and 300% by mass relative to the anionically modified CNFin the preparation of the dry solid.

Example 16 to 18

The same procedures as described in Examples 5, 12, and 13 wereperformed except that a carboxymethyl cellulose available under thebrand name F04HC having a viscosity of about 50 mPa·s (1%, 25° C.) and acarboxymethyl substitution degree of about 0.9 was used.

Example 19 to 21

The same procedures as described in Examples 5, 12, and 13 wereperformed except that a carboxymethyl cellulose available under thebrand name FT-3 having a viscosity of about 30 mPa·s (1%, 25° C.) and acarboxymethyl substitution degree of about 0.9 was used.

Examples 22 and 23

The same procedure as described in Example 5 was performed except thatpolyvinyl alcohol (PVA) (KURARAY POVAL 105 from KURARAY CO., LTD.) and astarch (potato starch from Hokuren Group) were used respectively inplace of the carboxymethyl cellulose.

Comparative Examples 1 and 2

The same procedures as described in Examples 5 and 6 were performedexcept that the carboxymethyl cellulose was not added in the preparationof the dry solid.

Comparative Example 3

The same procedure as described in Example 5 was performed except thatthe carboxymethyl cellulose was not added and the pH was not adjusted inthe preparation of the dry solid.

The results are shown in Table 1.

TABLE 1 Water-soluble polymer Dispersity Base Parts by Vigorous MildViscosity Transparency material pH Type mass stirring stirring recovery% recovery % Thixotropy Example 1 TOCNF 9 CMC-1 40 3 2 11 100 3 Example2 TOCNF 8 CMC-1 40 2 1 5 67 2 Example 3 TOCNF 11 CMC-1 40 3 2 10 92 3Example 4 TOCNF 11 CMC-1 100 3 2 9 100 2 Example 5 CMCNF 9 CMC-1 40 3 254 98 4 Example 6 CMCNF 8 CMC-1 40 3 2 24 93 4 Example 7 CMCNF 11 CMC-140 3 2 49 98 4 Example 8 CMCNF 9 CMC-1 5 2 1 59 96 4 Example 9 CMCNF 9CMC-1 20 2 2 58 97 4 Example 10 CMCNF 9 CMC-1 25 3 2 63 98 4 Example 11CMCNF 9 CMC-1 65 3 2 64 100 3 Example 12 CMCNF 9 CMC-1 100 3 3 75 100 3Example 13 CMCNF 9 CMC-1 150 3 3 100 100 3 Example 14 CMCNF 9 CMC-1 2003 3 100 100 3 Example 15 CMCNF 9 CMC-1 300 3 3 100 100 2 Example 16CMCNF 9 CMC-2 40 2 1 27 79 2 Example 17 CMCNF 9 CMC-2 100 3 2 40 81 2Example 18 CMCNF 9 CMC-2 150 3 3 65 98 2 Example 19 CMCNF 9 CMC-3 40 2 140 77 2 Example 20 CMCNF 9 CMC-3 150 3 2 43 83 2 Example 21 CMCNF 9CMC-3 100 3 3 70 83 2 Example 22 CMCNF 9 PVA 40 2 2 32 91 3 Example 23CMCNF 9 Starch 40 2 1 19 — 3 Comparative CMCNF 9 None 1 1 11 100 —example 1 Comparative CMCNF 8 None 1 1 — — — example 2 Comparative CMCNF7 None 1 1 10 83 — example 3

It is apparent from the results shown in Table 1 that redispersionimproves when dry solids of anionically modified CNFs are combined withwater-soluble polymers.

The invention claimed is:
 1. A process for preparing a dry solid of amixture of an anionically modified cellulose nanofiber and awater-soluble polymer, comprising 5 to 60% by mass of the water-solublepolymer relative to the anionically modified cellulose nanofiber,wherein the anionically modified cellulose nanofiber comprises an aniongroup obtained by oxidizing a hydroxyl group of a glucopyranose ring ofthe cellulose, or an anion group obtained by binding a hydroxyl group ofa glucopyranose ring of the cellulose with an anion group via an etherbond or an ester bond, wherein a moisture content of the dry solid is12% by mass or less, the process comprising: forming a film of anaqueous suspension containing the anionically modified cellulosenanofiber and the water-soluble polymer on a drum of a drum dryer or abelt of a belt dryer, wherein the film has a thickness of 50 to 1000 μm;supplying heat energy to the film to dry the film, and scraping thedried film on the drum or the belt off to obtain the dry solid.
 2. Theprocess according to claim 1, wherein the thickness of the dried film is100 to 300 μm.
 3. A process for preparing a dry solid of a mixture of ananionically modified cellulose nanofiber and a water-soluble polymer,comprising 5 to 60% by mass of the water-soluble polymer relative to theanionically modified cellulose nanofiber, wherein the anionicallymodified cellulose nanofiber comprises an anion group obtained byoxidizing a hydroxyl group of a glucopyranose ring of the cellulose, oran anion group obtained by binding a hydroxyl group of a glucopyranosering of the cellulose with an anion group via an ether bond or an esterbond, wherein a moisture content of the dry solid is 12% by mass orless, the process comprising: adjusting an aqueous suspension containingthe anionically modified cellulose nanofiber and the water-solublepolymer to pH 9 to 11; and then drying the aqueous suspension.